Retired UK special forces soldiers have reportedly volunteered for missions inside Ukraine. The veterans, aged between 40 and 60, have fought around the world including Afghanistan and Iraq.
Among them are highly-trained snipers and experts in the use of anti-aircraft and anti-tank missiles.
It is understood the operation will be funded by an unnamed European country via a private military company.
The plan is a way of backing Ukraine militarily without officially sending serving British troops.
More than a dozen of the veterans have reportedly already arrived in Ukraine and more will travel this week.
One ex-SAS soldier told the Daily Mirror: “This has had to happen very quickly as the situation in Ukraine is now desperate and they need urgent help.
“A lot of ex-parachute regiment colleagues are also very keen to go. Many people are very keen to go and it has had to be organised very quickly.
“It is legal as none of the men going out are still serving and Ukraine has invited people to go in and help the fight militarily.
“It means some of the most experienced veterans in the world will assist Ukraine and link up with Ukraine’s force headquarters.
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Asked on the BBC’s Sunday Morning programme if she would support UK citizens who chose to answer the call, Ms Truss said: “That is something people can make their own decisions about.
“The people of Ukraine are fighting for freedom and democracy, not just for Ukraine but for the whole of Europe.
“Absolutely, if people want to support that struggle I would support them in doing that.”
But Defence Secretary Ben Wallace on Monday urged Britons not to travel to Ukraine to join the fighting.
Mr Wallace said that while Ms Truss was right that it was a “just cause”, he did not “want to see British people killed any more than I want to see Ukrainians” dying.
He told Sky News: “If you’re keen to help and you’re a United Kingdom citizen, come and join our armed forces.
“Look, there are people who will go… I think what I would say is unless you are properly trained, unless you are a, you know, experienced member of an armed forces, I think there are better ways for you to contribute to the security of Ukraine.”
Mr Wallace said this could be through donating money through the Ukrainian embassy to be spent on weaponry and aid, or to volunteer with organisations helping refugees.
Later on Monday, the Prime Minister’s official spokesman said: “We think the best way we can help Ukraine right now is by ensuring Putin fails.
“There are a number of ways Brits can show their support for that, and the Ukrainian embassy in London is putting out information about how British people can support.
“We fully recognise the strength of feeling about British people wanting to support the Ukrainians following the Russian invasion.
“There’s advice up on travelling to Ukraine, we currently advise against travel to Ukraine.”
There have been questions about the legality of Britons going abroad to fight in previous foreign conflicts.
In 2014, the Crown Prosecution Service warned that UK nationals who went to fight in the Syrian civil war could be committing an offence, even if they joined the rebels fighting to overthrow President Bashar Assad.